Process control systems typically include one or more process controllers communicatively coupled to one or more field devices via analog, digital, or combined analog/digital buses. Information from the field devices and/or the controller is usually communicated over a data highway or communication network (e.g., HART protocol) to devices, such as operator workstations. These devices enable an operator to perform a variety of functions, including viewing the current state of a process of the process control system.
Traditionally, every instrument from which data is collected is individually wired to a Distributed Control System (DCS). Wiring costs can be several thousand dollars per instrument. In addition to the cost of the wire involved, which may be substantial, the cost of wiring an instrument to the DCS also includes the cost of the labor to install the wiring and the cost of obtaining the necessary certifications. Wiring certain instruments may be impractical when the data obtained from those instruments is not sufficiently critical to the operation of the process control system to justify the cost of the wiring. The inability to obtain data from certain locations in the process control system may result in not having information that may be potentially useful to the operator. For example, data, such as output pressure, is collected at a positioner, but that data does not necessarily correspond to the reaction of a valve coupled to that positioner, and there is no indication of the status of other nearby instruments. Any additional data the valve and other instruments might provide could be helpful to the operator in determining the state of the process control system.
Wireless transmission of data from instruments having non-critical locations in a process control system have been based on, for example, Wireless HART. However, the update rates using such communications are slow (e.g., once a minute) and not practical for use with data that is time dependent. Additionally, Wireless HART based communications require remote instruments or sensors to include batteries. However, batteries have to be replaced periodically due to the amount of power required to transmit the data and, thus, often involve a tradeoff between power and range. Using such known communications, an instrument typically transmits data a short distance to another transmitter, which then relays the data to subsequent transmitters until the data eventually reaches the DCS. However, using multiple instruments to relay data in this manner can be impractical in areas where no instrumentation is required.